The present invention relates to a bi-axial tilting specimen fine motion device in an electron microscope, and more particularly to a eucentric side entry bi-axial-tilting specimen fine motion device capable of automatically correcting image shifting during tilting a specimen.
A common bi-axial-tilting specimen fine motion device (goniometer stage) in an electron microscope is a eucentric side entry goniometer stage.
FIG. 3 is a cross-sectional view showing the outline of a eucentric side entry goniometer stage. A specimen 3 is mounted on a specimen holder 20 which becomes a specimen stage. The specimen holder 20 is inserted inside a fine motion cylinder 30 universally pivotable with a sliding surface of a spherical part 29 in a vacuum sealed state to a main body 28 by an O-ring 27 using a drive spring part 25, a horizontal drive screw 26, a drive spring part and a vertical drive screw, not shown, provided in positions in the direction perpendicular to the plane of the figure which are provided in a tilt base body 24 rotated by a tilt drive worm gear 23, and is loaded inside a space of an electron beam passage 32 in a vacuum sealed state by an O-ring 31. On the other hand, the specimen holder 20 inserted into the fine motion cylinder 30 is linked to a fine motion shaft 16 jointed to a horizontal drive shaft 13 slidable in a vacuum sealed state by an O-ring 33 as shown in the figure.
Therefore, when the horizontal drive shaft 13 is driven, the specimen 3 can be moved in the y-direction through the fine motion shaft 16 and the specimen holder 20. On the other hand, when the horizontal screw 26 is driven, the specimen 3 can be finely moved in the x-direction because the specimen holder 20 is pivotally moved together with the fine motion cylinder 30 about a center point K of the spherical part 29 which functions as a fulcrum of pivotal movement, i.e., a point of support, of the specimen holder 20. The top front portion of the specimen holder 20 contacting to the fine motion shaft 16 is of a spherical-shape having the point K as the center. The top front portion of the fine motion shaft 16 contacting to the specimen holder is of a spherical-shape having the contact portion of the horizontal drive shaft 13 and the fine motion shaft 16 as the center. Therefore, the top front portion of the specimen holder 20 and the top front portion of the fine motion shaft 16 always contact to one point on a tilt shaft 17 of the tilt base body 24 independently of the fine motion in the x-direction. Let the one point be a point P. The point P moves on the shaft 17 in the range of .+-.1 mm at maximum with movement of the horizontal drive shaft 13. Therefore, the radius of the sphere in the top front portion of the specimen holder 20 is formed to be equal to the distance between the point P and the point K when the point P is in the middle point of the range of its movement.
By driving the horizontal drive screw, not shown, the specimen holder 20 can be pivotally moved around the point K of the spherical portion 29 as a point of support through the fine motion cylinder 30 to finely move the specimen 3 in the z-direction.
Further, by rotating the tilt base body 24 with driving the tilt drive worm gear 23, the specimen 3 can be tilted to the optical axis in the .theta..sub.x -direction by rotating the specimen holder 20 through the fine motion cylinder 30.
In the specimen holder 20, as shown in a plan view of FIG. 4A and a cross-sectional view of FIG. 4B, there is installed a specimen table 35 rotatable around an axis intersecting the center axis of the specimen holder 20 (that is, the x-axis) at right angle. By rotating the specimen table 35 with an external operation, the specimen 3 can be tilted in the .theta..sub.y -direction.
The operational principle of a eucentric side entry goniometer stage will be described, referring to FIG. 5. The reference character 1 indicates the optical axis of an electron beam, and the reference character 3 indicates a specimen. An axis 17 indicates the tilt axis for the tilt angle .theta..sub.x in the x-direction. The axis 17 is adjusted so as to intersect the optical axis 1. A point A is the intersection and an observed point. The reference character 14 is the reference line for tilt angle .theta..sub.x of 0.degree.. An axis 12 is the center axis of the specimen holder 20 mounting the specimen 3. A point K is the intersection of the tilt axis 17 and the center axis of the specimen holder 12. When the axis 12 moves in the direction indicated by an arrow, the specimen 3 is moved toward the direction opposite to the arrow direction. An axis 13 is the axis of the specimen drive shaft in the y-direction, and substantially agrees with the axis 17.
The fine motion shaft 16 is a part to mechanically link the specimen drive shaft 13 in the y-direction with the specimen holder 20, and a driving force transmitting part to move the specimen 3 in the y-direction. The mechanical contact portion of the fine motion shaft 16 and the specimen holder 20 coincides with the point P in the tilt axis 17 as have been described in FIG. 3. By moving the center axis of the specimen holder 20 in the z-direction shown in the arrow, the specimen can be moved toward the direction opposite to the arrow direction perpendicular to the plane formed by the axis 15 and the axis 12. Further, the specimen 3 can be tilted around the tilt axis 15 for tilt angle .theta..sub.y in the y-direction as a rotating center axis, FIG. 5 shows a case where the tilt angle of .theta..sub.y in this direction is 0.degree. (zero degree). The axis 12 is a reference line for .theta..sub.y =0.degree..
In this case, by making the specimen superposed on the point A with adjustment of the z-direction, since the point A is on the tilt axis 17 for .theta..sub.x, the specimen does not shift from the observed point A even if the tilt angle .theta..sub.x is varied, that is, the specimen does not shift from the field of observation in principle. The point (0, 0) in the figure indicates the intersection of the axis 12 and the axis 15 and the point of origin of the coordinate system of x, y for small movement of the specimen. By making the specimen superposed on the point A through varying the small movement of the specimen x, y and adjusting a position in the z-direction in an arbitrary position, the specimen does not shift out of the field of observation even if the tilt angle .theta..sub.x is varied. This is the reason to be called as eucentric goniometer. This performance can be realized in that the tilt axis 17 for tilt angle .theta..sub.x can be set as a fixed axis independently of the movement of the specimen in the x-direction.
However, since the construction is formed such that the tilt axis 15 for tilt angle .theta..sub.y in the y-direction moves together with the specimen motion y, the tilt axis 15 cannot be made to coincide with the specimen at the point A even if adjusting in the z-direction is performed except when y=0, that is, the tilt axis 15 intersects the point A. Therefore, if the specimen is tilted in the .theta..sub.y -direction, the specimen shifts out of the field of observation.
In a bi-axial-tilting specimen fine motion device such as a eucentric side entry goniometer stage mounted in an electron microscope, a position being observed does not always agree with the specimen tilt axis. If the specimen is tilted during observation, the specimen position being observed will shift out of the field of observation.
Therefore, it is required to manually correct the displacement of the specimen every time the specimen is tilted while the image of the specimen is being observed using a specimen fine motion device; that is a very inconvenient operation.